The fundamental reaction $pp\to \{pp{\}}_s\gamma$, where $\{pp{\}}_s$ is a proton pair with low excitation energy, has been observed with the ANKE spectrometer at COSY-Jülich for proton beam energies of $T_p=0.353$, 0.500, and 0.550 GeV. This is equivalent to photodisintegration of a free ${}^{1}S_0$ diproton for photon energies $E_{\gamma }\approx T_p/2$. The differential cross sections measured for c.m. angles $0°<{\theta }_{pp}<20°$ exhibit a steep increase with angle that is compatible with $E1$ and $E2$ multipole contributions. The ratio of the measured cross sections to those of $np\to \gamma d$ is on the ${10}^{-3}–{10}^{-2}$ level. The increase of the $pp\to \{pp{\}}_s\gamma$ cross section with $T_p$ might reflect the influence of the $\Delta (1232)$ excitation.

• Received 3 June 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.102501